These days, an optical fiber communication system in which information can be transmitted with high speed as compared to a coaxial cable communication system has been introduced into a market to meet the demand of transmitting a large capacity of information. The modulation method presently used in an optical fiber communication system is an optical direct modulation method in which a drive current of a light source such as a semiconductor laser, etc. is modulated to directly modulate an intensity of an emitted light. In this direct modulation method, there is limitation in amount of information to be transmitted in a unit time, because the Chirping phenomenon is not negligible as a modulation frequency becomes demand of transmitting a much more increased amount of information, an optical modulator utilizing the electro-optic effect in which a refractive index of a material is changed by applying an electric field to the material has been researched and developed.
Although an optical modulator in which high speed modulation is possible to be carried out in principle, drift of voltage light output characteristics in which a light output waveform is distorted occurs as operation of the modulation continues. As a result, demodulation is difficult to provide a precise demodulated signal from a received light signal.
In order to cope with the disadvantage of drift as described above, a conventional optical modulation apparatus comprising a mean value detecting circuit for calculating a mean value of a modulated light from an optical modulator utilizing the electro-optic effect, a voltage comparing circuit for comparing the mean value with the reference voltage to provide a difference signal therebetween, and a bias voltage generating circuit for generating a bias voltage dependent on the difference signal has been proposed.
In the conventional optical modulation apparatus, the bias voltage generated dependent on the difference signal is added to a modulating signal, and an added signal thus obtained is supplied to the optical modulator. In this operation, the bias voltage is controlled to be coincided with an operative point voltage of the optical modulator to suppress the distortion in waveforms of output lights caused by the drift.
Other than the above described conventional optical modulation apparatus, the Japanese Patent Kokai No. 59-17527 has described a bias voltage control apparatus for an optical modulator comprising a circuit for detecting a difference of duty-cycles between an input modulating signal and a modulated light, and a circuit for controlling a bias voltage dependent on the difference to make the bias voltage coincident with an operative point voltage of the optical modulator. In addition, the Japanese Patent Kokai No. 56-165122 has described a method for preventing DC drift from occurring in an optical modulator or an optical polarizer by dividing a buffer layer, and the Japanese Patent Kokai No. 58-68250 has described an optical modulation apparatus in which an output light of an optical modulator and a smoothed signal of the output light are compared to control a bias voltage dependent on a difference therebetween.
In the conventional optical modulation apparatus, however, there is a disadvantage in that the modulation operation is required to be temporally suspended for the re-setting of a bias voltage prior to the occurrence of the situation where a controlled bias voltage is over a break-down voltage of the optical modulator. This results in the consequence that the modulation operation continues successively for 10 years of an operation warrant period for an optical communication apparatus. The technologies of the above described Japanese Patent Kokais result in the similar disadvantages.
In order to overcome the disadvantage of the break-down voltage, there has been proposed a method in which a bias voltage is re-set to be a lower one by subtracting value corresponding to one period from a higher one, when the bias voltage is over a predetermined value, because voltage light output characteristics are periodical in regard to voltage. Even in this method, there is the same disadvantage in that the modulation operation is temporality suspended at the re-setting time of the bias voltage.